U.S. patent number 6,958,868 [Application Number 10/813,749] was granted by the patent office on 2005-10-25 for motion-free tracking solar concentrator.
Invention is credited to John George Pender.
United States Patent |
6,958,868 |
Pender |
October 25, 2005 |
Motion-free tracking solar concentrator
Abstract
An integrated solar concentrator and tracker is constructed from
a beam deflector for unpolarized light in combination with a fixed
optical condenser. The one-dimensional beam deflector consists of a
pair of prism arrays made from a material whose refractive index
can be varied by applying an electric field. Two of the
one-dimensional concentrators can be arranged with their faces in
contact and with their prism arrays perpendicular to construct a
two-dimensional beam deflector. The intensity and distribution of
an applied field modifies the refractive index of the individual
prisms in order to keep direction of the deflected beam fixed as
the incident beam shifts. When the beam deflector is used with the
fixed concentrator the result is that the position of the focus
remains fixed as the source moves.
Inventors: |
Pender; John George (Fairbanks,
AK) |
Family
ID: |
35115327 |
Appl.
No.: |
10/813,749 |
Filed: |
March 29, 2004 |
Current U.S.
Class: |
359/742; 126/569;
136/246; 359/484.01 |
Current CPC
Class: |
F24S
23/00 (20180501); F24S 23/10 (20180501); F24S
23/31 (20180501); H01L 31/0543 (20141201); H01L
31/0547 (20141201); G02B 5/045 (20130101); F24S
50/20 (20180501); G02F 1/29 (20130101); G02F
1/133526 (20130101); Y02E 10/47 (20130101); Y02E
10/44 (20130101); Y02E 10/52 (20130101) |
Current International
Class: |
G02B
3/08 (20060101); G02B 5/30 (20060101); G02B
27/28 (20060101); F24J 2/00 (20060101); H01L
25/00 (20060101); G02B 003/08 (); G02B 005/30 ();
G02B 027/28 (); F24J 002/00 (); H01L 025/00 () |
Field of
Search: |
;359/625-627,726,727,742,743,495,265,266
;126/569,571-573,598-700,684 ;136/246 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Epps; Georgia
Assistant Examiner: Harrington; Alicia M.
Claims
What is claimed is:
1. An optical concentrator for concentrating light from a moving
light source comprising: a) a substantially transparent sheet made
from a material having a substantially constant refractive index;
b) wherein said substantially transparent sheet has a series of
grooves formed in a face, said series of grooves defining a groove
array; c) wherein said groove array is filled with a layer of
substantially transparent material having an electrically
changeable refractive index when an electromagnetic field is
applied thereto, said layer of substantially transparent material
being defined as an active layer; d) a means for applying an
electromagnetic field having a changeable strength to said active
layer; e) a means for controlling the strength of said
electromagnetic field; f) an optical condenser, positioned in
optical communication with said substantially transparent sheet and
said active layer; g) wherein light from a light source interacts
with said substantially transparent sheet and said active layer and
said optical condenser such that said light is concentrated in a
localized region of space.
2. The optical concentrator of claim 1 further comprising: a) a
plurality of substantially transparent sheets having a plurality of
groove arrays; b) a plurality of active layers; c) wherein said
substantially transparent sheets and said active layers are
positioned in optical communication with said optical
condenser.
3. The optical concentrator of claim 2 wherein an active layer lies
between two conducting layers that are electrically connected to a
voltage source for providing an electromagnetic field through said
active layer when said voltage source produces a voltage.
4. The optical concentrator of claim 3 wherein said plurality of
groove arrays have grooves that are parallel with respect to one
another.
5. The optical concentrator of claim 4 wherein an active layer is
comprised of a liquid crystal material having a director.
6. The optical concentrator of claim 5 wherein said plurality of
active layers has a first group of directors having directors that
are parallel with respect to one another, and a second group of
directors having directors that are perpendicular with respect to
the directors of said first group of directors.
7. The optical concentrator of claim 3 wherein said plurality of
groove arrays has a first group of groove arrays having grooves
that are parallel with respect to one another, and a second group
of groove arrays having grooves that are perpendicular with respect
to the grooves in said first group of groove arrays.
8. The optical concentrator of claim 7 wherein an active layer is
comprised of a liquid crystal material having a director.
9. The optical concentrator of claim 8 wherein said plurality of
active layers has a first group of directors having directors that
are parallel with respect to one another, and a second group of
directors having directors that are perpendicular with respect to
the directors of said first group of directors.
10. The optical concentrator of claim 1 further comprising a
reflecting surface positioned in optical communication with said
substantially transparent sheet and said active layer and said
optical condenser, wherein light from a light source interacts with
said substantially transparent sheet and said active layer and said
optical condenser and said reflecting surface such that said light
is concentrated in a localized region of space.
11. The optical concentrator of claim 10 further comprising: a) a
plurality of substantially transparent sheets having a plurality of
groove arrays; b) a plurality of active layers; c) wherein said
substantially transparent sheets and said active layers are
positioned in optical communication with said optical condenser and
said reflecting surface.
12. The optical concentrator of claim 11 wherein an active layer
lies between two conducting layers that are electrically connected
to a voltage source for providing an electromagnetic field through
said active layer when said voltage source produces a voltage.
13. The optical concentrator of claim 12 wherein said plurality of
groove arrays have grooves that are parallel with respect to one
another.
14. The optical concentrator of claim 13 wherein an active layer is
comprised of a liquid crystal material having a director.
15. The optical concentrator of claim 14 wherein said plurality of
active layers has a first group of directors having directors that
are parallel with respect to one another, and a second group of
directors having directors that are perpendicular with respect to
the directors of said first group of directors.
16. The optical concentrator of claim 12 wherein said plurality of
groove arrays has a first group of groove arrays having grooves
that are parallel with respect to one another, and a second group
of groove arrays having grooves that are perpendicular with respect
to the grooves in said first group of groove arrays.
17. The optical concentrator of claim 16 wherein an active layer is
comprised of a liquid crystal material having a director.
18. The optical concentrator of claim 17 wherein said plurality of
active layers has a first group of directors having directors that
are parallel with respect to one another, and a second group of
directors having directors that are perpendicular with respect to
the directors of said first group of directors.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
Not Applicable
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable
DESCRIPTION OF ATTACHED APPENDIX
Not Applicable
FIELD OF THE INVENTION
This invention relates generally to the field of solar power and
more specifically to a machine for concentrating solar
radiation.
DESCRIPTION OF THE PRIOR ART
A number of systems for passive, or non-tracking, concentration of
solar energy in one dimension to a line-like focus have been
produced in the past. Among such systems are those shown in U.S.
Pat. Nos. 6,467,916, 5,537,991, 5,289,356, 4,487,961, 4,359,265,
4,230,095, 4,003,638, and 4,002,499. A common characteristic of
these systems is the use of smooth reflective surfaces to reflect
light from the sun onto a region to be heated, such as a
fluid-filled conduit.
The overall efficiency of the heating process is improved if the
system incorporates a tracking mechanism to compensate for the
apparent daily and seasonal motion of the sun. This can be
accomplished by keeping the solar collector fixed and moving the
target to the optical focus as the focus shifts with the motion of
the sun (such as U.S. Pat. No. 4,000,734). More commonly the
collector is continuously repositioned in order to keep the
location of the focus fixed as the sun moves. This can be
accomplished by adjusting the tilt of a mirror or an array of
mirrors (such as U.S. Pat. Nos. 4,148,564 and 4,355,630), or
adjusting the tilt of a prism or an array of prisms (such as U.S.
Pat. Nos. 4,377,154 and 4,382,434).
Applications such as daylighting require high concentration ratios
(point, or spot focus) and the ability to track the sun along two
axes. A wide range of collectors is used to accomplish this, from
Fresnel lenses and arrays of Fresnel lenses, to parabolic
reflectors and arrays of parabolic reflectors, to holographic
optical elements, and to combinations of these elements such as
those shown in U.S. Pat. Nos. 6,384,320, 6,299,317, 6,274,860,
5,325,844, 4,832,002, 4,409,963, 4,297,000, and 4,153,474. In all
cases it is necessary to incorporate a mechanical tracking
mechanism to keep the solar energy focused onto the target to
compensate for the sun's apparent daily and seasonal motion.
Existing solar collectors that actively track the sun's apparent
motion incorporate a mechanical tracking system that both supports
and tilts the collector in order to keep the solar energy aligned
with the target. The drive must be robust enough to move the solar
concentrator yet not be moved by external influences like the wind,
which has the effect of degrading the alignment. Potential failure
of the motion drive presents reliability and maintenance issues. As
the collector is made larger for higher power systems its increased
mass requires more robust support and tracking infrastructure,
which leads to even higher total mass. At some point it makes sense
to replace a single large collector with an array of smaller
collectors, each of which has its own tracking mechanism, but the
large number of mechanical drives compromises maintenance and
reliability.
U.S. Pat. No. 6,169,594 describes a device which steers a
collimated, polarized beam of light in one or two dimensions. A
control voltage varies the refractive index of one or more arrays
of prism-shaped cells of liquid crystal, which varies the
deflection angle of the incident beam of light. This beam steering
technology has not to date been generalized to steer unpolarized
light nor has it been used in conjunction with a focusing element
to concentrate light. The objective has been to create a small,
fast, device that steers an unexpanded, polarized laser beam with a
minimum of wavefront distortion. However, the problem of
concentrating solar radiation does not require high speed nor does
it require a high-quality image at the target. It does, however,
require good efficiency, the ability to handle unpolarized light,
large collection area, and a focusing element, none of which are
features of the current technology.
BRIEF SUMMARY OF THE INVENTION
The invention is a solar collector consisting of a Fresnel lens or
other optical condenser, and one or more arrays of prismatic cells
made from a material whose refractive index may be varied by
applying an electromagnetic field. The refractive index of the
cells is varied by the applied field in such a manner as to direct
sunlight at a fixed angle into the condenser which then
concentrates the light to a focus. The field is adjusted as the sun
moves in the sky in order to keep the position of the optical focus
fixed. There are no moving parts. Since the concentrator does not
need to move, the design and construction of a support structure
which resists vibration from wind is easier. Elimination of a
mechanical tracking mechanism improves reliability and reduces
overall weight. Other objects and advantages of the present
invention will become apparent from the following descriptions,
taken in connection with the accompanying drawings, wherein, by way
of illustration and example, an embodiment of the present invention
is disclosed.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view of the one-dimensional solar
collector using a Fresnel lens as the optical condenser.
FIG. 2 is a cross-sectional view of the one-dimensional solar
collector using a parabolic trough mirror as the optical
condenser.
FIG. 3 is a diagram illustrating Snell's Law.
FIG. 4a is an isometric view of a triangular prism.
FIG. 4b is a cross-sectional view of a triangular prism
illustrating the deflection of a beam of light.
FIG. 5a is an exploded isometric view showing a groove in a block
of transparent material and the material filling the groove.
FIG. 5b is an isometric view of a longitudinal director applied to
a prism-shaped volume of liquid crystal.
FIG. 5c is an isometric view of a transverse director applied to a
prism-shaped volume of liquid crystal.
FIG. 6 is an isometric view showing a first electrode configuration
which produces an electric field through a prism-shaped volume of
liquid crystal.
FIG. 7a is a cross-sectional view of a volume of liquid crystal
showing the orientation of liquid crystal molecules in the absence
of an electric field.
FIG. 7b is a cross-sectional view of a volume of liquid crystal
showing the orientation of liquid crystal molecules in the presence
of a moderate electric field.
FIG. 7c is a cross-sectional view of a volume of liquid crystal
showing the orientation of liquid crystal molecules in the presence
of a high electric field.
FIG. 8a is a cross-sectional view showing the maximum deflection of
an extraordinary ray incident on an asymmetric liquid crystal
prism.
FIG. 8b is a cross-sectional view showing the minimum deflection of
an extraordinary ray incident on an asymmetric liquid crystal
prism.
FIG. 9a is a cross sectional view showing the maximum deflection of
an extraordinary ray incident on a symmetric liquid crystal
prism.
FIG. 9b is a cross sectional view showing the minimum deflection of
an extraordinary ray incident on a symmetric liquid crystal
prism.
FIG. 10 is an isometric view of a second geometric configuration of
a liquid crystal prism embedded in a transparent block.
FIG. 11 is a cross-sectional view of a second electrode
configuration which produces an electric field through a liquid
crystal prism.
FIG. 12 is a cross-sectional view of a third electrode
configuration which produces an electric field through a liquid
crystal prism.
FIG. 13a is a cross-sectional view of positive lensing in a liquid
crystal prism due to nonuniform field.
FIG. 13b is a cross-sectional view showing positive lensing
corrected by concave groove curvature.
FIG. 14a is a cross-sectional view of negative lensing in a liquid
crystal prism due to nonuniform field.
FIG. 14b is a cross-sectional view showing negative lensing
corrected by convex groove curvature.
FIG. 15a is an exploded isometric view of a steering grating and
the material which is used to fill the grooves.
FIG. 15b is an isometric view of an assembled steering panel.
FIG. 16a is a diagram illustrating the measurement of surface
conductivity of a square piece of a material.
FIG. 16b is a diagram illustrating the measurement of surface
conductivity of two square pieces of a material placed in
series.
FIG. 16c is a diagram illustrating the measurement of surface
conductivity of two square pieces of a material placed in
parallel.
FIG. 16d is a diagram illustrating the measurement of surface
conductivity of four square pieces of a material arranged in the
form of a larger square.
FIG. 17a is an isometric view of the components used to construct a
first implementation of the 1D tracking solar concentrator.
FIG. 17b is an isometric view of a first implementation of the 1D
tracking solar concentrator.
FIG. 18a is an isometric view of the components used to construct a
second implementation of the 1D tracking solar concentrator.
FIG. 18b is an isometric view of a second implementation of the 1D
tracking solar concentrator.
FIG. 19a is an isometric view of the components used to construct a
third implementation of the 1D tracking solar concentrator.
FIG. 19b is an isometric view of a third implementation of the 1D
tracking solar concentrator.
FIG. 20a is an isometric view of the components used to construct a
fourth implementation of the 1D tracking solar concentrator.
FIG. 20b is an isometric view of a fourth implementation of the 1D
tracking solar concentrator.
FIG. 21a is a cross-sectional view of a fifth implementation of the
1D tracking solar concentrator.
FIG. 21b is an enlarged cross-sectional view of a portion of a
fifth implementation of the 1D tracking solar concentrator.
FIG. 22a is an isometric view of the components used to construct a
sixth implementation of the 1D tracking solar concentrator.
FIG. 22b is an isometric view of a sixth implementation of the 1D
tracking solar concentrator.
FIG. 23 is a cross-sectional view of a seventh implementation of
the 1D tracking solar concentrator.
FIG. 24a is an isometric view of the components used to construct
an eighth implementation of the 1D tracking solar concentrator.
FIG. 24b is an isometric view of an eighth implementation of the 1D
tracking solar concentrator.
FIG. 25a is a cross-sectional view of a pair of 1D tracking solar
concentrators used together in a segmented structure in order to
extend the deflection range.
FIG. 25b is an isometric view of a pair of 1D tracking solar
concentrators used together in a segmented structure in order to
extend the deflection range.
FIG. 26 is a cross-sectional view showing a 1D tracking solar
concentrator and a means by which the device may be controlled.
FIG. 27a is an isometric view of the components used to construct a
first implementation of the 2D tracking solar concentrator.
FIG. 27b is an isometric view of a first implementation of the 2D
tracking solar concentrator.
FIG. 28a is an isometric view of the components used to construct a
second implementation of the 2D tracking solar concentrator.
FIG. 28b is an isometric view of a second implementation of the 2D
tracking solar concentrator.
FIG. 29 is a cross-sectional view of a third implementation of the
2D tracking solar concentrator.
FIG. 30 shows a means by which the 2D tracking solar concentrator
may be controlled.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Introduction
The collection and concentration of solar energy is a fundamental
component in many heating, power generation, and daylighting
applications. A first application is solar water heating. This is
often accomplished by circulating water in a length of conduit,
both ends of which connect to a holding tank. Solar radiation is
concentrated on a section of the conduit, which gradually warms the
water in the tank as the water in the conduit circulates.
A second application is the production of electric power using
photovoltaic cells. Many applications involve a large-area device
which collects the radiation and concentrates it onto a small
photovoltaic cell which is specially designed to handle high power
density. The point is to lower overall cost by reducing the area of
expensive photovoltaic cells by incorporating relatively low-cost
solar concentrators.
A third application is fiberoptic daylighting. This refers to the
process of collecting visible radiation from the sun and directing
it into one end of an optical fiber. The other end of the fiber
terminates at a diffuser inside a building where the solar
radiation provides passive lighting.
A fourth application is mechanical power generation by
concentrating solar energy onto the heat cup of a Stirling engine.
The heat is converted into rotational mechanical energy which may
by used directly or converted to electricity using a generator.
All of these applications may be realized using the device
described in this document. FIG. 1 illustrates in cross section the
preferred implementation of a device which concentrates light to a
line-like focus for solar water heating. There is the beam
deflector assembly 31 and there is the optical condenser 32, shown
as a Fresnel lens. An electromagnetic field is applied to the beam
deflector assembly which changes the refractive index of
prism-shaped cells of liquid crystal, steering incoming light so
that it always strikes the Fresnel lens at a fixed angle. The
condenser then concentrates the light onto a stationary target such
as a length of fluid-filled pipe 34. The combined effect of these
elements is to keep light concentrated onto the pipe as the Sun
moves. Detectors 36 placed near the target provide feedback which
is used to maintain the quality of the alignment. FIG. 2 shows an
alternate implementation in which a parabolic trough reflector 38
is used instead of a Fresnel lens.
Analysis of a Triangular Prism
FIG. 3 is a diagram showing the refraction that takes place when
light strikes an interface 40 separating two different media, each
of which has a distinct refractive index. The relationship between
the direction of the incident ray 42 and that of the refracted ray
44 is quantified in an equation called Snell's Law, which may be
written
where n.sub.1 and n.sub.2 are the refractive indices of medium one
and medium two, respectively. FIG. 3 illustrates the case for which
n.sub.1 >n.sub.2, though the reverse may also be assumed. The
angle .theta. is defined with respect to a line 46 drawn
perpendicular to the interface 40 and through the point at which
the incident ray strikes the interface. Note that a ray striking a
surface at normal (perpendicular) incidence (.theta..sub.1 =0) is
not deflected.
Consider a triangular prism 48 embedded in a uniform medium as
shown in perspective in FIG. 4a and in cross section in FIG. 4b.
Rays of light incident on this prism always emerge from the prism
with their propagation direction changed if the refractive index of
the prism, n.sub.p, is different from the refractive index of the
surrounding medium, n.sub.m. The relevant task at this point is to
choose the propagation direction of the incident ray 42 in order to
ensure that the ray 44 emerging from the prism does so
perpendicular to the exit face 50 of the prism. Applying Snell's
Law to the incident face 52 leads to
where the angle between the incident face 52 and the exit face 50
of the prism is .alpha.. The angle between the incident ray and the
exit ray, referred to as the deflection angle, .phi., is thus
Assume that the refractive index of the prism, n.sub.p, may be
varied continuously over a range without changing the physical
dimensions or orientation of the prism. That is,
There is then a continuous range of incident angles at which an
incident ray 42 may strike the prism and be made to emerge from the
prism normal to the exit face 50 by adjusting the refractive index
of the prism. The deflection angle associated with the maximum
index, n.sub.p,max, is
and the deflection angle associated with the minimum index,
n.sub.p,min, is
The total range of deflection angles is .DELTA..phi.=.phi..sub.max
-.phi..sub.min.
FIG. 5a shows how such a prism may be constructed by filling a
prism-shaped groove in a block of transparent material 54 with a
material 56 whose refractive index may be changed by applying an
electromagnetic field. Ideally, a material which interacts in the
same way with all incident light polarizations is used to fill the
groove. The prism-shaped groove may also be filled with a liquid
crystal material. Liquid crystals comprise a class of materials
composed of elongated, polar molecules which tend to align with
their long axes parallel to one another. Typically, the surface of
a volume of liquid crystal is brushed to align the long axes of the
surface molecules in a specific direction. This has the effect of
aligning the long axes of all the other molecules in the same
direction, which is called the director. For the prism-shaped
volume of liquid crystal it is most convenient to set the director
in the plane of the exposed surface, either parallel to the long
dimension of the prism (longitudinal director) as is FIG. 5b, or
perpendicular to the long dimension (transverse director) as in
FIG. 5c.
Light polarized parallel to the director experiences a different
refractive index than light polarized perpendicular to the
director. This effect is referred to as birefringence and can be
used to make a prism with a variable refractive index for light
polarized parallel to the director. This is accomplished by
applying an electric field normal to the exit face of the prism as
shown in FIG. 6; i.e. parallel to the propagation direction of the
light and perpendicular to either director. A coating of conductor
58 thin enough to be nearly transparent is applied to the top and
the bottom surfaces of the prism/block device, and a voltage is
applied between the conducting surfaces. This creates an electric
field 60 in the desired direction whose strength depends on the
magnitude of the applied voltage.
FIG. 7a shows the orientation of liquid crystal molecules 62 in a
volume of liquid crystal material in the absence of an applied
electric field. The director is well defined. In FIG. 7b a voltage
is applied to transparent conducting layers 58 on the top and
bottom surfaces. The resulting electric field 60 causes the
molecules to rotate away from the director and toward the direction
of the applied field. The stronger the field, the greater the
alignment with the field. Eventually, as shown in FIG. 7c, almost
all the molecules are aligned with the field (the surface molecules
being the exception) and not the director. At this point increasing
the field no longer changes the optical properties of the liquid
crystal. When the field is turned off the molecules return to their
original orientation.
Unpolarized light 63 incident on the volume of liquid crystal may
be resolved into a component polarized parallel to the director,
referred to as the extraordinary ray, and a component polarized
perpendicular to the director, referred to as the ordinary ray. The
polarization of the ordinary ray remains perpendicular to the long
axis of the molecules for all values of the applied voltage so it
experiences no change in refractive index, which is referred to as
the ordinary refractive index, n.sub.o. The polarization of the
extraordinary ray is parallel to the long axis of the molecules
when there is no field (FIG. 7a) and the refractive index it
experiences in this case, n.sub.e, is typically larger than
n.sub.o. As the applied electric field increases (FIG. 7b) the long
axis of the molecules rotates away from the polarization direction
of the extraordinary ray, which changes the refractive index the
light experiences. When all the molecules are aligned with the
applied field the extraordinary ray experiences the same refractive
index as the ordinary ray, n.sub.o, as shown if FIG. 7c. Therefore
the refractive index of the extraordinary ray can be controlled by
an applied voltage and lies in the range
By way of example, consider the device illustrated in cross section
in FIG. 8. Assume that the transparent embedding block 54 is made
of polycarbonate, n.sub.poly =1.58, and the prism-shaped volume of
liquid crystal 56 has .alpha.=45.degree.. BDH-E44 is a typical
liquid crystal, having ordinary refractive index n.sub.o =1.53 and
zero-field extraordinary refractive index n.sub.e =1.79. This and
other liquid crystals are commercially available from E. M.
Industries of Hawthorne, N.Y., as well as other suppliers. Snell's
Law is used to calculate the deflection of an incident beam of
light, though it is now necessary to account for two interfaces:
the boundary 64 between air and the liquid crystal, and the
boundary 66 between the liquid crystal and the polycarbonate. As
before, it is assumed that the ray strikes the exit face of the
device (the bottom surface of the polycarbonate block) normal to
its surface. The deflection angle for the ordinary ray is a
constant, calculated for refractive index n.sub.o, and has value
.phi..sub.min =-2.9.degree.. This angle is also one extreme of the
range of possible incident angles for the extraordinary ray. The
other extreme is calculated for refractive index n.sub.e and has
value .phi..sub.max =11.5.degree.. The deflection range of the
extraordinary ray,
.DELTA..phi.=13.1.degree.+2.8.degree.=14.4.degree., can be
increased in at least two ways. First, a different liquid crystal
with a larger birefringence, n.sub.e -n.sub.o, may be used. Second,
the angle .alpha., may be increased. For instance, if
.alpha.=60.degree. instead of 45.degree. then the deflection angles
become .phi..sub.min =-5.2.degree. and .phi..sub.max =18.4.degree.,
for a deflection range of .DELTA..phi.=23.6.degree..
A case may be made for changing the cross section of the prism as
shown in FIG. 9. The symmetry of the groove allows the deflection
range to extend from -.phi..sub.max to .phi..sub.max for a total
deflection range of .DELTA..phi.=2 .phi..sub.max. Using the numbers
from the previous example, this becomes .DELTA..phi.=23.0.degree.
for .alpha.=60.degree. and .DELTA..phi.=36.8.degree. for
.alpha.=60.degree.. The down side is that only half the
polycarbonate-liquid crystal surfaces are participating in the
process at any given time.
It is worthwhile examining the embedded prism with a slightly
different geometry, as shown in FIG. 10, in which the exit face of
the prism, not the entrance face of the prism, is parallel to the
faces of the embedding block. The exit ray is assumed to emerge
normal to the exit face of the prism, the materials in the previous
example are reused, and Snell's Law is used to calculate the
trajectory of incident rays. When a .alpha.=45.degree. the
deflection angles become .phi..sub.min =-2.8.degree. and
.phi..sub.max =13.1.degree., for a total deflection range of
.DELTA..phi.=15.9.degree.. When .alpha.=60.degree. the deflection
angles become .phi..sub.min =-4.7.degree. and .phi..sub.max
=30.7.degree., for a total deflection range of
.DELTA..phi.=35.4.degree.. The deflection range is considerably
larger than that of the previous example, despite the fact that the
prism has exactly the same dimensions, because the orientation of
the media with respect to the incident light have an effect on the
refraction of light. The deflection range can also be affected, for
instance, by relaxing the condition that the exit ray must be
normal to the bottom surface.
From these simple examples it is clear there are many design
parameters which influence device performance: choice of materials,
especially the liquid crystal, and the orientation and apex angle
of the prism. In general, the design of a device which maximizes
total energy in a day (solar heating) will be different than a
design which maximizes deflection range (passive solar
lighting).
Curved Grating Faces
There are at least three ways to generate the applied electric
field through the liquid crystal prism. The first method has been
shown in FIG. 6, wherein an essentially transparent coating 58
which is conductive enough to form an equipotential surface has
been applied to the top and bottom surfaces of the prism/block
device. The second method is to apply the coating 58 to the
interface between the liquid crystal and the fixed index material
as well as the face containing the exposed liquid crystal surface,
as shown in FIG. 11. The third method is to use a transparent
conducting material 68 for the fixed index block as shown in FIG.
12. The latter two methods have the advantage of not having to
create a field in the fixed refractive index material in order to
get a field in the liquid crystal, which reduces the total required
voltage. The latter two methods also have a disadvantage in that
the two conducting surfaces are now very close together. Should the
two surfaces come into electrical contact the electric field would
disappear entirely.
The optical analysis performed above on the prisms presumes a
uniform electric field applied to the liquid crystal prisms. There
is only one way to do this: the electrode configuration of FIG. 6
and the requirement that the liquid crystal and the fixed index
medium have the same electric permittivity. If this is not the
case, or if either the second or third electrode configuration is
used, then the electric field will vary from one part of the liquid
crystal prism to another. This leads to a spatial nonuniformity in
the extraordinary refractive index, the general effect of which
will be that the light emerging from the prism will not be
collimated; i.e. "lensing" will take place.
To some degree the lensing effect may be counteracted by using
curved refracting surfaces. If the lensing acts like a positive
lens, as in FIG. 13a, then a concave groove would recollimate the
light as shown in FIG. 13b. If the lensing acts like a negative
lens as in FIG. 14a, then a convex groove would recollimate the
light as shown in FIG. 14b. If this feature is to be implemented
then the liquid crystal and the transparent block material should
be chosen to make n.sub.o and n.sub.block as close as possible to
minimize lensing of the ordinary ray. While it is doubtful that one
could perfectly compensate for index nonuniformity in this way, it
could improve performance significantly and it doesn't cost any
more to produce than a triangular grating once a die or mold has
been fabricated.
1-Dimensional Solar Collector--Basic construction
For the 1-dimensional case a particular application concentrates
solar energy to a line focus for solar water heating. The
discussion above indicates that such a device may be built, in
part, from a series of parallel prism-shaped volumes of a material
whose refractive index may be controled by an applied electric
field. One way to construct such a device is to start with a
steering grating 70 stamped, cast or machined into a sheet of
transparent material such as polycarbonate or acrylic as shown in
FIG. 15a. The simplest grating resembles a sawtooth in cross
section. The tooth spacing and the angle a may be fixed or these
parameters may be varied in a specific application. Filling the
grooves of the steering grating with a material whose refractive
index can be varied in a controlable manner results in an array of
individual volumes 72, wherein each volume is shaped like the
triangular prism analyzed above. Ideally, the material is
isotropic. The steering grating 70 together with the material which
fills the grooves 72 are referred to as a steering panel 76 as
shown in FIG. 15b.
There are two issues to be addressed if the device is to use a
liquid crystal material to fill the grooves. First, liquid crystal
is birefringent and only the extraordinary refractive index may be
varied by applying a field, so the single steering panel of FIG.
15b will only actively steer one polarization component of incident
light. This may be be sufficient for a particular application.
Should the application demand steering both polarizations the
solution is to use a pair of prism arrays, one on top of the other,
in which the director of the first array is perpendicular to the
director of the second array.
The second issue is the fact that the extraordinary refractive
index of liquid crystals varies under the influence of an applied
electric field. This field is created by introducing a thin layer
of conducting material on either side of each layer of liquid
crystal and applying a voltage between the conducting planes. There
are two basic criteria for the thin conducting layer: it must be
transparent (or nearly so) and it must be a conductor. It need not,
however, be a good conductor. The motion of the sun demands a very
modest response time from the liquid crystal, which means the
conductive layer need only be conductive enough to produce an
equipotential surface under nearly steady conditions. A static
dissipative (SD) layer, with a surface resistivity in the range
10.sup.6 -10.sup.8 Ohms/square, conducts well enough to remove
accumulated charge in less than 50 milliseconds. A layer designated
as conductive (10.sup.3 -10.sup.6 Ohms/square) is a better
conductor but is typically less transparent than a static
dissipative layer.
Ohms per square is the unit of surface resistivity. It is a
material parameter like density which is independent of size.
Consider the measurement of surface resistance of square piece 75
of a given material as shown in FIG. 16a. Suppose for the sake of
argument that the surface resistance is 1 Ohm. A pair of identical
squares in series, shown in FIG. 16b, will have a total surface
resistance of 2 Ohms. A pair of identical squares in parallel,
shown in FIG. 16c, will have a total surface resistance of 1/2 Ohm.
Finally, four identical squares arranged into a square as shown in
FIG. 16d will have a surface resistance of 1 Ohm, identical to that
of the single square. Therefore, measurement of the surface
resistance of a material is independent of the area as long as the
material is in the shape of a square.
Sheets of various thicknesses of clear acrylic, polycarbonate or
polyvinyl chloride (PVC) with one or both faces coated with an SD
layer are available commercially from a number of manufacturers,
such as Boedeker Plastics of Shiner, Tex., Terra Universal of
Anaheim, Calif. and SciCron Technologies of Amarillo, Tex.
Alternatively, a roll of film with an SD coating may be purchased
from the same suppliers. In the simplest implementation, the
steering grating is constructed from sheet material which has a
static dissipative layer 74 on one surface as in FIG. 15a.
With these conditions in mind, several possible solutions present
themselves. The preferred implementation is constructed from two
steering panels 76 and a 1-dimensional Fresnel lens 32, as shown in
FIG. 17a, all of which have a transparent conducting layer 74 of at
least static dissipative quality on the smooth surface. The focal
length of the Fresnel lens depends on the application and the focus
may lie on a line perpendicular to the surface of the Fresnel lens
and passing through the center of the lens, or it may be offset.
Many Fresnel lenses are available commercially from companies such
as Fresnel Technologies.
Assembly begins with a first steering grating placed smooth surface
down. The grooves of the steering grating are filled with liquid
crystal material and the surface is brushed to fix the director
either parallel to the long axis of the prism cells (longitudinal
director) or perpendicular to the long axis (transverse director).
A second steering grating is placed, smooth surface down, directly
atop the first steering grating with the grooves of the second
grating parallel to the grooves of the first grating. The grooves
of the second steering grating are filled with liquid crystal and
the surface is brushed to fix the director perpendicular to the
director of the first layer of liquid crystal. Finally, the Fresnel
lens is placed, smooth surface down, directly on top of the second
steering grating. If a 1-dimensional Fresnel lens is used the the
grooves of the Fresnel lens must be parallel to the grooves in the
steering gratings. The entire assembly is then flipped over as
shown in FIG. 17b. Light enters from above and is concentrated to a
line below the assembly. The layers may be cemented together or
pressed together while the edges are sealed.
A second preferred implementation uses a parabolic trough mirror 38
instead of a Fresnel lens, as shown in FIG. 18b. The conducting
surface formerly provided by the Fresnel lens can be provided by an
unmachined piece 78 of the material used to make the steering
gratings, referred to as a cover plate, which has a static
dissipative layer 74 on one face as shown in FIG. 18a. The
transparent conducting surface of the cover plate is in contact
with the liquid crystal prisms.
There are many variations on this theme, combinations of steering
gratings and cover plates used with either a parabolic trough, a
1-dimensional Fresnel lens, a 2-dimensional Fresnel lens, or some
other optical condenser as shown in FIGS. 19 and 20. The two
steering panels 76 in FIG. 20 are separated by a thin film 80 that
has a conductive coating. Either the lower steering panel has the
conductive coating (as shown in FIG. 20a) or the smooth side of the
Fresnel lens has the conductive coating, but not both.
The variations shown in FIG. 21 and FIG. 22 operate in a somewhat
different way than the previous implementations. In both cases,
light passes through the steering panel assembly, strikes a mirror
and then passes through the steering panel assembly a second time
as it travels to the focus. In FIG. 21 the mirror is a parabolic
trough 38 and doubles as the condenser. The curved steering panels
84 are applied to the inner surface of the mirror. In FIG. 22 a
refracting condenser such as a Fresnel lens 32 is used and the
mirror 82 is flat. All that is necessary to produce the flat mirror
is to replace the transparent conductive layer of the bottom most
steering grating with a reflective conductive coating such as a
thin layer of aluminum. This device is particularly interesting:
since the focus is located above the Fresnel lens it can be made
very thin and laid flat on any smooth, opaque surface such as the
pitched roof 85 of a house as shown in FIG. 23. Since the device no
longer needs to support its own weight the steering panels may be
made very thin, which saves on material cost. Furthermore, the
addition of a lightweight 1D convex reflector 87 suspended above
the device allows the target, normally a pipe filled with
circulating fluid, to rest on the surface of the Fresnel lens.
In all cases, the throughput and deflection range are determined by
the steering gratings, and the concentration ratio is determined by
the condenser. In all cases the placement of the three conducting
layers is such that one may separately control the extraordinary
refractive index of each array of liquid-crystal-filled prisms
should this be necessary. A voltage between the top conducting
layer and the middle conducting layer produces an electric field in
the upper array of liquid crystal prisms, and a voltage between the
middle conducting layer and the bottom conducting layer produces an
electric field in the lower array of liquid crystal prisms.
Analyze the steering panel of FIG. 17 as a representative case. The
construction and analysis for all the other implementations
proceeds in much the same way. The steering gratings can be made
from 3 mm thick polycarbonate which has one static dissipative
surface (available from Boedeker Plastics, Inc.). Use a blaze angle
of .alpha.=45.degree. degrees, and a groove density of 2
grooves/mm. A variety of standard liquid crystals are available, of
which BDH-E44 is a typical member, having ordinary refractive index
n.sub.o =1.53 and zero-field extraordinary refractive index n.sub.e
=1.79. This and other liquid crystals are commercially available
from E. M. Industries of Hawthorne, N.Y. The steering assembly is
fastened onto a Fresnel lens, or across the face of a parabolic
trough for which the focus lies just inside the face. Solar energy
is concentrated to a line focus onto a water line as shown in FIG.
1 and FIG. 2. Analysis using Snell's Law indicates that the
incident angles for which this particular beam deflector can
provide normal incidence on the condenser ranges from -5.57.degree.
to 9.74.degree. for one polarization and from -5.57.degree. to
9.70.degree. for the other polarization. When a is increased to
60.degree. then the incident angles for which this particular beam
deflector can provide normal incidence on the condenser ranges from
-9.2.degree. to 20.0.degree. for one polarization and from
-9.2.degree. to 19.5.degree. for the other polarization, for a
total deflection range of about 29.degree..
Since there is a difference, however modest, between the steering
of one polarization versus the other, a separate voltage control
must be provided for each liquid crystal layer for maximum quality
of focus. In practice the voltage differences are slight. If the
somewhat reduced performance is acceptable the device may be
simplified by having a single voltage source control both liquid
crystal layers.
A deflection range of 29.degree. is acceptable for a line focus
solar water heater designed to accommodate for the Sun's seasonal
variation in temperate latitudes. A simple calculation shows that a
properly aligned device (for which one extreme of the deflection
range corresponds to the summer solstice) accommodates the position
of the Sun for 105 days on either side of the summer solstice (210
days total). That is, the device works 7 months of the year, from
early March until early October. In milder climates, where solar
water heating is an option for larger portions of the year, a
device with a larger deflection range may be designed and
constructed. For instance, using symmetric grooves as in FIG. 9 and
.alpha.=60.degree. leads to a deflection range of about 40.degree.,
which would accommodate the position of the Sun for 136 days on
either side of the summer solstice (from early February to early
November).
The deflection range can be improved in several ways. First, a
liquid crystal which has larger birefringence may be used. Second,
a larger blaze angle may be used. Third, symmetric grooves may be
used as shown in FIG. 9 and discussed above. Fourth, two or more of
the beam deflectors as described above may be stacked one on top of
the other (i.e. four or more steering panels) as shown in FIGS. 24a
and 24b. Fifth, one can make a segmented structure consisting of
two or more panels that are tilted with respect to each other as
shown in FIGS. 25a and 25b. These variations may be used in any
combination depending on the details of the application.
Alignment and Control
There are two basic strategies one may pursue for maintaining the
optical alignment of the device. In either case, control of the
refractive index of the liquid crystal prisms requires a variable
voltage source and a power supply. The first strategy uses a clock
and a look-up table. If the latitude and alignment (usually
East-West) of the device are known then the position of the Sun can
be calculated for any time of day or year. A table yields the
voltage required to keep the focus at the desired location. This is
simple conceptually, but in practice this approach is complicated
by changing temperatures, shifting of the physical components of
the device over time, and wind loading.
FIG. 26 illustrates a more sophisticated strategy wherein detectors
36 are placed near the desired location of the focus. The output of
the detectors is used to continuously monitor the quality of the
alignment so the control voltage(s) may be adjusted as necessary.
In the preferred implementation a pair of photovoltaic (PV) cells
36 is placed behind a target 34 such as a water pipe. The PV cells
are placed side by side with their adjacent edges parallel to and
directly behind the target. Each PV cell is wired 86 into a
controlable voltage supply 88 which provides a voltage to each
steering panel via electrical connections 90. Most of the
concentrated light falls on the pipe while a small amount misses
the pipe and strikes one or both PV cells. If the power absorbed by
one cell is the same as that absorbed by the second cell then the
bulk of the optical power falls on the pipe as desired. If one PV
cell absorbs more power than the other the device is presumed to be
misaligned. The output of the voltage supply is adjusted until the
powers from the PV cells equalize. As an added feature, the PV
cells can be used to power the device; the liquid crystal layers
function electrically as capacitors with very low leakage current
so they require very little power. The PV cells and the voltage
supply can be manufactured as a unit, and the voltage is supplied
to the conductive layers of the beam steering assembly with wires.
Controlling the tracking solar concentrator in this way makes the
device less susceptible to alignment degradation due to shifting of
the components, and active feedback mitigates the problems caused
by wind noise.
Other components may be added to improve the control of the device.
For instance, a rechargeable battery may be used to power the
device, so that alignment is preserved during periods of cloud
cover and darkness. The pair of PV cells may then be used to power
a recharging circuit for the battery as well as to provide
alignment status to the voltage control.
Two Dimensional Device
Certain applications, such as daylighting via fiberoptic cable,
require high concentration of light, i.e. point focus as opposed to
line focus. This in turn requires two-dimensional tracking of the
sun's apparent motion. The preferred 2D implementation uses a
point-focus condenser, such as the Fresnel lens 92 shown in FIG.
27a, and at least two pairs of steering panels. Each pair has a
steering panel with a transverse director and a steering panel with
a longitudinal director. The grooves of the first pair of steering
panels are perpendicular to the grooves of the second pair of
steering panels. Construction proceeds in the same way as the
one-dimensional device.
The 1D variations illustrated in FIGS. 17 through 24 can all be
generalized to the 2D case. Of particular interest is the 2D
version of FIG. 22, shown in FIG. 28. Once again, the bottom most
steering grating has a reflective conductive coating 82 instead of
a transparent conductive coating, and the Fresnel lens 92 and all
the steering gratings 76 can be made from very thin material. Since
the focus lies above the Fresnel lens the device may then lie flat
on a smooth opaque surface and requires no other mechanical
support. FIG. 29 shows this device mounted to a pitched roof 85.
The addition of a lightweight convex reflector 89 suspended above
the device allows the target, such as the end of an optical fiber
91, to reside on the surface of the Fresnel lens.
Alignment of the 2D optical concentrator can be controlled with a
square array of photovoltaic cells 94 surrounding the end of the
fiber 96 as shown in FIG. 28. Comparison of the total power in cell
A and cell B to the total power in cell C and cell D are used to
maintain alignment of the Sun in the north-south direction.
Comparison of the total power in cell A and cell C to the total
power in cell B and cell D are used to maintain alignment of the
Sun in the east-west direction.
While the invention has been described in connection with a
preferred embodiment, it is not intended to limit the scope of the
invention to the particular form set forth, but on the contrary, it
is intended to cover such alternatives, modifications, and
equivalents as may be included within the spirit and scope of the
invention as defined by the appended claims.
* * * * *